Question: Some gorillas came on time to school this morning. $10$ gorillas came late. There were $20$ gorillas in school today. How many gorillas came on time?
Explanation: $ \begin{aligned}&{?} \text{ gorillas on time}\\ +&\underline{ {10} \text{ gorillas came late}}\\ =&{20} \text{ gorillas in school}\end{aligned}$ ${10}$ gorillas came on time.